1. Field of the Invention
The invention relates to porous medium sensors for measuring parameters or properties in porous media and, more particularly, it relates to a porous medium sensor having a sensing portion insertable in the porous medium with a reference port in gas communication with ambient air.
2. Description of the Prior Art
Porous medium sensors for monitoring parameters in soil are known. For example, tensiometers monitor matrix water potential ψm (or soil moisture tension), which is an indirect measure of soil water content. Tensiometers are used in irrigation scheduling to help farmers and other irrigation managers to determine when to water. Tensiometers can also be used in the scientific study of soil and plant behaviors.
Typically the porous medium sensors are partially inserted in growing media, such as soil, for monitoring purposes. They include a sensing portion which is insertable in the growing medium and a head which extends outwardly. A section of the housing can also extend outwardly of the growing medium.
Several porous medium sensors, such as tensiometers, have a reference port in gas communication with ambient air. The reference port can be either immersed in the growing medium or located outwardly of the growing medium when a sensing portion of the porous medium sensor is inserted therein. Water and porous medium infiltration into the reference port, can bias measurements taken in the growing medium, even if the latter is located above the growing medium. To prevent this problem, it is known to cover the reference port with a water-repellent membrane [See for instance U.S. patent application no. 2010/0263436 filed on Apr. 30, 2010].
However, the water-repellent membrane can seal in heavy growing media, for instance the ones having high clay content. Furthermore, if the water-repellent membrane is inserted in a fine growing media, a trapped volume of air can surround the water-repellent membrane and also bias the soil property measurements. Furthermore, water-repellent membranes do not perfectly repel water in all conditions, particularly if the soil becomes hydrophobic. In particular conditions, water and/or small particles can thus infiltrate the sensor through the reference port.